1. Field of the Invention
The present invention relates to a method for correcting an instrumental error, mainly due to a wavelength error of a spectroscope or spectrometer which is provided with or built in an optical analyzer for measuring a content, a concentration or an activity of an analyte by measuring an optical transmitting or reflection density of a dry analysis element.
2. Description of the Related Art
The dry analysis element is an analysis element which comprises one or plural functional layers and an analytical reagent composition is contained in at least one (or plural) of these functional layers so as to form a coloring dye by the reaction taking place in the layer. The thus formed dye is colorimetrically analyzed by measuring the transmitting or reflection light from the outside of the analysis element. Since such a dry analysis element is stored and preserved in the dry state prior to the analysis operation, there is no need for preparing the reagent at the measuring step. Furthermore, since the reagent has a higher stability in the dry condition, the process using such a dry analysis element is improved in simplicity and quickness of operation over the conventional wet process in which the reagent solution should be prepared as required. The dry process or method using the dry analysis elements has become rapidly a popular analytical method which can easily realize an automated analyzing system in the field of clinical examinations where a number of test samples are to be handled as routine works.
For a quantitative analysis, a liquid solution containing the analyte is spotted or applied onto the dry analysis element and an optical density of a transmitting light through or a reflected light from the analysis element after the completion of the coloring reaction is measured by optical density measuring instruments such as a colorimeter and a spectrophotometer. In such a method, generally, it is common practice to determine the content of the analyte by using a standard curve (commonly referred to as "calibration curve") which has been preliminary drawn by plotting the interrelation between the known contents of the analyte in the standard samples and the optical transmitting or reflection densities of the analysis elements to which standard samples are applied.
However, there is a problem such that an instrumental error is caused due to variation or deviation of the optical density measuring apparatus as used. That is, even when analysis elements of the same manufacturing lot are used, measured values obtained by different optical analyzer may be fluctuated. The optical density measuring apparatus spectroscopically divides the irradiating light or the measuring light through spectroscopes such as a diffraction grating or an interference filter into a monochromatic light of specified wavelength, and is used to measure the optical density of light at the specified wavelength. The accuracy of measurement depends on the performance (accuracy of the spectral centroid of the measured central wavelength, spectral band width and peak width at half height of the spectrum) of the spectroscope. Particularly, the interference filter type spectrophotometer has no adjustability of the spectral centroid and spectral band width of measured light, therefore, the performance of this spectroscope is a cause of the instrumental error. When the absorption wavelength spectrum of a subject to be measured, i.e., the formed dye produced in the analysis element, is relatively moderate pattern and the optical density at the wavelength peak (or trough) of this spectrum is to be measured, a slight error of the measured wavelength and the half-amplitude level hardly affects the measurement value (optical density) obtained and the instrumental error cannot be a serious problem. However, if a slope portion of the absorption wavelength spectrum of the subject to be measured is the wavelength to be spectrophotometrically measured, a measurement value (optical density value) to be obtained fluctuates even with a slight deviation of the measured central wavelength and the half-amplitude level, and therefore the error or difference between measuring instruments cannot be ignored.
Therefore, in most cases, the standard or calibration curve is prepared for each measurement and each measuring apparatus to ensure the accuracy of quantitative analyses. However, a number of standard samples must be prepared and examined to draw a standard curve, it is troublesome to prepare these standard samples for each assay, and the quickness of analyses is impaired.
To avoid the complexity of such drawing of the standard curve for each analysis, a few method has been proposed wherein a sample having known content or concentration is measured by the measuring apparatus to detect an error or difference of the measurement so that the instrumental error of the measuring apparatus is corrected or compensated. For example, the instrumental error correcting method for the optical density measuring apparatus which uses a correction or compensation plate (the standard colored plate) has been proposed in the Unexamined Japanese Patent Publication No. 60447/1992. In the method disclosed in the Unexamined Japanese Patent Publication No. 60447/1992, a dry analysis element contains either of the same dye as the final product (formed dye) of the measuring system or a colored matter having an absorption spectrum similar to that of the formed dye in the measuring wavelength range, and such dry analysis element is used as a compensation plate or colored standard plate. In details, an optical density (OD.sub.0) is obtained by measuring a sample through an interference filter, which is the reference filter, and on the other hand, an optical density value (OD.sub.x) is obtained by measuring the same sample through a specific interference filter, which is a target of compensation and then the interrelation between these optical density values is represented as a primary function shown by the following formula (1) and correction coefficients k and j of this correction formula (1) are evaluated. EQU OD.sub.0 =k.times.OD.sub.x +j (1)
To obtain these correction coefficients k and j, a plurality of correction plates (standard colored plates) having different optical density values (typically, optical density of a reflected light) are prepared to be measured through both the reference interference filter and a specific interference filter to be compensated, and a primary regression analysis is conducted from the OD values obtained from these filters. Since the correction plates to be used in this method contain the dye or coloring matter having the absorption spectrum as same as or similar to that of the final product (formed dye) in the analysis elements, the method using such a correction plate is more excellent in accuracy of the correction of the measurement values in comparison with the conventional method using monochrome filters as the correction plates.
However, since the correction method disclosed in the Unexamined Japanese Patent Publication No. 60447/1992 uses two variables as correction coefficients, at least two correction plates (standard colored plates) having different optical densities are required to be measured. Further, more correction plates would be required because these correction coefficients are actually evaluated by the primary regression equation. To prepare these correction plates, the final product (formed dye) of the measuring assay system or the dye having a similar absorption spectrum should be contained in advance in the dry analysis element, and therefore such analysis element for use as the correction plates should be prepared, in addition to the dry analysis element for analysis of the analyte. If the final product (formed dye) of the measuring assay system is unstable and is not durable, a colored matter, which has a similar absorption spectrum of the final product (formed dye) within the measuring wavelength range or region, should be separately provided to prepare a suitable correction plate.
Furthermore, it has become to be clarified that the effect of correction is low, when a blank value of the reagent is high, that is, the analysis element which has not reacted before application of the sample solution has high optical density as a background or reagent blank, or when the optical density after the completion of the coloring reaction is low.